1. Technical Field
The present invention relates to a crane, in particular to a revolving tower crane, having a boom rotatable about an upright slewing gear axis by a slewing gear drive and having an out-of-operation brake which allows and brakes rotary movements of the boom in the out-of-operation state.
2. Description of Related Art
The boom in revolving tower cranes, but also in other crane types, is rotatable about an upright slewing gear axis, with a slewing gear provided for this purpose being able to have a rotary drive, for example in the form of an electric motor, whose drive movement is converted into a rotary movement of the boom via a slewing gear transmission, for example in the form of a planetary gear. In this respect, in so-called top-slewers, the boom is rotated relative to the tower supporting the boom, whereas in so-called bottom slewers, the entire tower together with the boom supported thereat is rotated relative to the undercarriage or support base.
In crane operation, the rotary movements are controlled by a corresponding control of the rotary drive, with a slewing gear brake being provided for braking and also for a rotational fixing in a specific rotary position. Such slewing gear brakes can typically be configured for safety reasons such that the brake is preloaded into its braking operation position, for example by a corresponding spring device, and can be released by an adjustment actuator to release the rotatability.
In non-operation, or in the out-of-operation state when the crane is shut down, it is, however, desirable that the crane can rotate to be able to align itself with wind in the most favorable rotary position with respect to the respective wind direction. Since, for example, revolving tower cranes are typically much more stable, due to their ballast load, against tilt movements in the boom plane than in with respect to tilt movements transversely to the boom planes passing through the boom in a perpendicular manner, the crane should align itself under a strong wind such that the wind comes from behind and the boom is aligned with respect to the wind as parallel as possible with the direction of the wind since otherwise there would be a risk of a tilt of the crane or the crane would have to have additional ballast. To allow such an automatic alignment in the wind, a wind release apparatus is connectable/can be connected with the service brake or slewing gear brake and releases the brake, which is typically preloaded into its braking position, when the crane is out of operation. This “end of work” position of the slewing gear brake can be set by means of a manually actuable adjustment lever, but optionally also by a powered release drive which can move the brake actuator into a locked non-braking position before the crane is powered down. Document EP 14 22 188 B1, for example, shows such a wind release apparatus for the slewing gear brake of a revolving tower crane.
The free rotatability of the crane in the out-of-operation state can, however, result in instabilities of the crane due to self-rotation under unfavorable wind conditions. If the crane is, for example, between two buildings and only the boom or only the counter-boom is exposed to the wind, only the boom or only the counter-boom is respectively flowed against at one side by the wind, whereby the crane can be set into ever faster rotation since the crane does not come to a standstill when the boom has turned out of the wind or before the counter-boom has moved into the wind. The boom and the counter-boom can hereby alternately move into the wind so that a build-up of this cyclic wind action can result in an auto-rotation of the crane which causes the crane to rotate too fast and to tilt.
To avoid such an unwanted auto-rotation, it has already been proposed not to let the slewing gear rotate fully unbraked in the out-of-operation state, but rather to associate an additional brake with the slewing gear which admittedly allows the rotary movement of the crane under wind, but slightly brakes it to defuse the aforesaid auto-rotation problem. It has, for example, been considered to provide a light out-of-operation brake at the outlet of the slewing gear transmission which applies a limited braking torque against the crane rotation which is smaller than the torque produced by the wind action so that the crane can still align itself in the wind, but can only rotate at a low rotational speed.
Such an additional brake is, however, difficult to configure with respect to the braking torque to be equally suitable for different wind conditions and also for different crane positions. For example, too high a braking torque can have the result under a moderate wind that the crane does not align itself properly, while the same braking torque cannot sufficiently suppress said auto-rotation under very unfavorable wind conditions at high wind speeds. In addition, with revolving tower cranes having a luffable boom, the luffing position in which the crane was shut down can have an influence on the required braking torque.